Viral infection induces the activation of the innate immune and Type I interferon responses, which act to restrict viral replication and spread. West Nile virus (WNV) is an emerging mosquito-borne pathogen that can cause fatal encephalitis in multiple vertebrate animal species. In the majority of cases, activation of the cell-intrinsic and cell-extrinsic innate immune responses controls viral replication and prevents encephalitis and death. Understanding the mechanisms by which WNV infection is inhibited by the innate immune response requires an understanding of the antiviral functions of innate immune proteins induced by infection. In this proposal, we will characterize the antiviral action of an evolutionarly conserved family of proteins termed interferon-induced protein with tetratricopeptide repeats (IFIT). IFIT proteins are induced early and to high levels during an innate immune response and have been shown to recognize viral RNA as non-self due to the presence of atypical RNA modifications at the 5' end; however, the mechanism by which this recognition occurs and the role of individual IFIT proteins in recognizing viral RNA has yet to be determined. Here, we will use biochemical and cell-culture based assays to determine the ability of IFIT proteins to bind viral RNA and determine how RNA binding and recognition inhibits viral replication. We also will use a mouse model lacking all three Ifit genes to assess tissue- and cell-type-specific effects of IFIT antiviral activity. We hypothesize that different combinations of IFIT proteins will differentially affect viral replication and that IFIT proteins will show a tissue- and cell type-specific dominance. The results of this study may support the use of viruses from multiple families that are highly susceptible to IFIT antiviral activity as novel vaccines and provide an avenue for development of novel antiviral agents that sensitize viruses to the antiviral effects of IFIT proteins.